Electroluminescent device, and display device comprising the same

ABSTRACT

An electroluminescent device comprising a first electrode and a second electrode facing each other, an emission layer disposed between the first electrode and the second electrode and including at least two light emitting particles, a hole transport layer disposed between the first electrode and the emission layer, and an electron transport layer disposed between the emission layer and the second electrode, wherein the electron transport layer comprises an inorganic layer disposed on the emission layer, the inorganic layer comprising a plurality of inorganic nanoparticles; and an organic layer directly disposed on at least a portion of the inorganic layer on a side opposite the emission layer, wherein a work function of the organic layer is greater than a work function of the inorganic layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2018-0028836 filed in the Korean IntellectualProperty Office on Mar. 12, 2018, and all the benefits accruingtherefrom under 35 U.S.C. § 119, the entire content of which isincorporated herein by reference.

BACKGROUND 1. Field

An electroluminescent device and a display device comprising thereof aredisclosed.

2. Description of the Related Art

Quantum dots are a nanocrystal semiconductor material having a diameterof about several to several hundreds of nanometers (nm) that provides alarge surface area per unit volume, and exhibit quantum confinementeffects. Quantum dots generate stronger light in a narrower wavelengthregion than the commonly used phosphor. Quantum dots emit light whilethe excited electrons are transited from a conduction band to a valenceband and wavelengths are changed depending upon a particle size, evenwithin the same material. The quantum dot particle sizes can be selectedto obtain light in a desirable wavelength region.

In other words, the emission layer including quantum dots, and thevarious types of electronic devices including the same, may generallyreduce production cost, compared to the use of organic light emittingdiodes having an emission layer including a phosphorescence and/orphosphor material. In addition, a desirable color may be emitted bychanging sizes of quantum dots, rather than having to use other organicmaterials in the emission layer for emitting other colors of light.

The luminous efficiency of the emission layer including quantum dots isdetermined by the external quantum efficiency (EQE) of quantum dots,which is determined based on a balance of charge carriers, lightextraction efficiency, and the like. Accordingly, when an emission layerincluding a quantum dot is applied as an electro-luminescence layer,improving the luminous efficiency of the emission layer requiresadjusting the balance of charge carriers and light extractionefficiency, together with reducing current leakage that can beassociated with the use of various charge carrier layers.

SUMMARY

An electroluminescent device capable of minimizing a leakage currentwhile improving a charge carrier balance and light extraction efficiencyof an emission layer is provided.

According to an embodiment, an electroluminescent device includes afirst electrode and a second electrode facing each other; an emissionlayer disposed between the first electrode and the second electrode andincluding at least two light emitting particles; a hole transport layerdisposed between the first electrode and the emission layer; and anelectron transport layer disposed between the emission layer and thesecond electrode, wherein the electron transport layer includes aninorganic layer disposed on the emission layer, the inorganic layercomprising a plurality of inorganic nanoparticles; and an organic layerdirectly disposed on at least a portion of the inorganic layer on a sideopposite the emission layer, wherein a work function of the organiclayer is greater than a work function of the inorganic layer.

The organic layer may completely cover an upper surface of the inorganiclayer between the inorganic layer and the second electrode.

The organic layer may have a lowest unoccupied molecular orbital (LUMO)energy level of about −1.8 electron Volts (eV) to about −2.8 eV.

The organic layer may have an electron mobility of about 10⁻³ squarecentimeters per Volt-second (cm²/V·s) to about 10⁻¹ cm²/V·s.

An inorganic nanoparticle of the plurality of inorganic nanoparticlescan be ZnO, TiO₂, ZrO₂, SnO₂, WO₃, Ta₂O₃, or a combination thereof.

An average particle diameter of the plurality of inorganic nanoparticlesmay be less than or equal to about 150 nanometers (nm).

The inorganic layer may be disposed on the emission layer as a clusterlayer comprising the plurality of the inorganic nanoparticles.

The cluster layer may be disposed directly on the emission layer.

The upper surface of the cluster layer may include a surface regionhaving two or more grains each comprising the inorganic nanoparticles,wherein a grain boundary is formed between adjacent grains, and theorganic layer may fill at least a portion of the grain boundary.

The organic layer may include an organic semiconductor compound that isa quinolone-based compound, a triazine-based compound, a quinoline-basedcompound, a triazole-based compound, a naphthalene-based compound, or acombination thereof.

The organic layer may include at least two different organicsemiconductor compounds.

The organic layer may include a first organic semiconductor compound anda second organic semiconductor compound that are different from eachother and a weight ratio of the first organic semiconductor compound andthe second organic semiconductor compound in the organic layer may beabout 3:7 to about 7:3.

An average thickness of the organic layer may be about 2 nm to about 20nm.

The electron transport layer may further include at least one additionalunit comprising a second inorganic layer and a second organic layer,wherein the at least one additional unit may be alternately stacked.

The plurality of light emitting particles may include quantum dots.

The quantum dots may be a Group II-VI compound that does not include Cd,a Group III-V compound, a Group IV-VI compound, a Group IV element orcompound, a Group I-III-VI compound, a Group I-II-IV-VI compound thatdoes not include Cd, or a combination thereof.

The quantum dots may have a core-shell structure.

The electron transport layer may not have electroluminescence.

According to another embodiment, a display device includes theelectroluminescent device.

An electroluminescent device having driving characteristics andlife-span characteristics by improving a charge carrier balance andlight extraction efficiency of an emission layer and simultaneouslyminimizing a leakage current and a display device including the same maybe provided.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects will become apparent and more readilyappreciated from the following description of the embodiments, taken inconjunction with the accompanying drawing, in which:

FIG. 1 is a schematic cross-sectional view of an electroluminescentdevice according to an embodiment,

FIG. 2 is a scanning electron microscope (SEM) image showing the uppersurface of the inorganic layer of the electroluminescent devicesaccording to an embodiment,

FIG. 3A is a three-dimensional contour image of an upper surface of aninorganic layer according to an embodiment as measured using a Zygointerferometer,

FIG. 3B is a graph of height (nanometers, nm) versus distance(millimeters, mm) for the linear portion indicated in FIG. 3A,

FIG. 4 is a schematic energy band diagram of an electroluminescentdevice according to an embodiment,

FIG. 5 is a graph of current density (milliamperes per squarecentimeter, mA/cm²) versus Voltage (volts, V) showing a voltage-currentdensity relationship of the electroluminescent devices according toExample 1 and Comparative Examples 1 to 3,

FIG. 6 is a graph of luminescence (candela per square meter, cd/m²)versus Voltage (V) showing a voltage-luminance relationship of theelectroluminescent devices according to Example 1 and ComparativeExamples 1 to 3,

FIG. 7 is a graph of external quantum efficiency (EQE, %) versusluminescence (cd/m²) showing a luminance-external quantum efficiencyrelationship of the electroluminescent devices according to Example 1and Comparative Examples 1 to 3,

FIG. 8 is a graph of current density (mA/cm², logarithmic scale) versusVoltage (V) showing a voltage-current density relationship of theelectroluminescent devices according to Example 1 and ComparativeExamples 1 to 3,

FIG. 9 is a graph of luminance (%) versus time (hour, h) showing atime-luminance relationship of the electroluminescent devices accordingto Example 2, Comparative Example 1, and Comparative Example 2,

FIG. 10 is a graph of current density (mA/cm²) versus Voltage (V)showing a voltage-current density relationship of the electroluminescentdevices according to Example 3 and Comparative Example 4,

FIG. 11 is a graph of luminescence (cd/m²) versus Voltage (V) showing avoltage-luminance relationship of the electroluminescent devicesaccording to Example 3 and Comparative Example 4,

FIG. 12 is a graph of external quantum efficiency (EQE, %) versusluminescence (cd/m²) showing a luminance-external quantum efficiencyrelationship of the electroluminescent devices according to Example 3and Comparative Example 4, and

FIG. 13 is a graph of current density (mA/cm², logarithmic scale) versusVoltage (V) showing a voltage-current density relationship of theelectroluminescent devices according to Example 3 and ComparativeExample 4.

DETAILED DESCRIPTION

Example embodiments of the present disclosure will hereinafter bedescribed in detail, and may be understood by a person skilled in theart. However, this disclosure may be embodied in many different forms,and is not to be construed as limited to the example embodiments setforth herein. If not defined otherwise, all terms (including technicaland scientific terms) in the specification may be defined as commonlyunderstood by one skilled in the art. The terms defined in agenerally-used dictionary may not be interpreted ideally orexaggeratedly unless clearly defined. In addition, unless explicitlydescribed to the contrary, the word “comprise” and variations such as“comprises” or “comprising”, will be understood to imply the inclusionof stated elements but not the exclusion of any other elements. The term“a combination thereof” is open ended and means including at least oneof the listed components, and may include other like components.

Further, the singular includes the plural unless mentioned otherwise.

Accordingly, the embodiments are merely described below, by referring tothe figures, to explain aspects of the present description. As usedherein, the term “and/or” includes any and all combinations of one ormore of the associated listed items. The term “or” means “and/or.”Expressions such as “at least one of” when preceding a list of elements,modify the entire list of elements and do not modify the individualelements of the list.

In the drawings, the thickness of layers, films, panels, regions, etc.,are exaggerated for clarity. Like reference numerals designate likeelements throughout the specification. It will be understood that whenan element such as a layer, film, region, or substrate is referred to asbeing “on” another element, it can be directly on the other element orintervening elements may also be present. In contrast, when an elementis referred to as being “directly on” another element, there are nointervening elements present.

It will be understood that, although the terms first, second, third,etc. may be used herein to describe various elements, components,regions, layers and/or sections, these elements, components, regions,layers and/or sections should not be limited by these terms. These termsare only used to distinguish one element, component, region, layer orsection from another element, component, region, layer, or section.Thus, a first element, component, region, layer, or section discussedbelow could be termed a second element, component, region, layer, orsection without departing from the teachings of the present embodiments.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used herein, thesingular forms “a,” “an” and “the” are intended to include the pluralforms as well, unless the context clearly indicates otherwise.

“About” or “approximately” as used herein is inclusive of the statedvalue and means within an acceptable range of deviation for theparticular value as determined by one of ordinary skill in the art,considering the measurement in question and the error associated withmeasurement of the particular quantity (i.e., the limitations of themeasurement system).

Exemplary embodiments are described herein with reference to crosssection illustrations that are schematic illustrations of idealizedembodiments. As such, variations from the shapes of the illustrations asa result, for example, of manufacturing techniques and/or tolerances,are to be expected. Thus, embodiments described herein should not beconstrued as limited to the particular shapes of regions as illustratedherein but are to include deviations in shapes that result, for example,from manufacturing. For example, a region illustrated or described asflat may, typically, have rough and/or nonlinear features. Moreover,sharp angles that are illustrated may be rounded. Thus, the regionsillustrated in the figures are schematic in nature and their shapes arenot intended to illustrate the precise shape of a region and are notintended to limit the scope of the present claims.

Relative terms, such as “lower” or “bottom” and “upper” or “top,” may beused herein to describe one element's relationship to another element asillustrated in the Figures. It will be understood that relative termsare intended to encompass different orientations of the device inaddition to the orientation depicted in the Figures. For example, if thedevice in one of the figures is turned over, elements described as beingon the “lower” side of other elements would then be oriented on “upper”sides of the other elements. The exemplary term “lower,” can therefore,encompasses both an orientation of “lower” and “upper,” depending on theparticular orientation of the figure. Similarly, if the device in one ofthe figures is turned over, elements described as “below” or “beneath”other elements would then be oriented “above” the other elements. Theexemplary terms “below” or “beneath” can, therefore, encompass both anorientation of above and below.

In an embodiment, for sizes or particle diameters of various particles,although they may be numerized by a measurement to show an average sizeof a group, the generally used method includes a mode diameter showingthe maximum value of the distribution, a median diameter correspondingto the center value of integral distribution curve, a variety of averagediameters (numeral average, length average, area average, mass average,volume average, etc.), and the like. Unless particularly mentioningotherwise, average sizes or average particle diameters means to numeralaverage sizes or numeral average diameters in the present disclosure,and it is obtained by measuring D50 (particle diameters at a position ofdistribution rate of 50%).

As used herein, “Group” in the term Group III, Group II, and the likerefers to a group of Periodic Table.

As used herein, “Group II” refers to Group IIA and Group IIB, andexamples of a Group II metal include Cd, Zn, Hg, and Mg, but are notlimited thereto.

As used herein, examples of “Group II metal that does not include Cd”refers to a Group II metal except Cd, for example Zn, Hg, Mg, etc.

As used herein, “Group III” refers to Group IIIA and Group IIIB, andexamples of Group III metal include Al, In, Ga, and TI, but are notlimited thereto.

As used herein, “Group IV” refers to Group IVA and Group IVB, andexamples of a Group IV metal include Si, Ge, and Sn, but are not limitedthereto. As used herein, “metal” may include a semi-metal such as Si.

As used herein, “Group I” refers to Group IA and Group IB, and examplesinclude Li, Na, K, Ru, and Cs, but are not limited thereto.

As used herein, “Group V” refers to Group VA, and examples includenitrogen, phosphorus, arsenic, antimony, and bismuth, but are notlimited thereto.

As used herein, “Group VI” refers to Group VIA, and examples includesulfur, selenium, and tellurium, but are not limited thereto.

Hereinafter, a work function (WF), a highest occupied molecular orbital(HOMO) energy level, and a lowest unoccupied molecular orbital (LUMO)energy level can be expressed as an absolute value from a vacuum energylevel (i.e., 0 electron volts (eV)). In addition, when the workfunction, HOMO energy level, or LUMO energy level is said to be ‘deep,’‘low,’, or ‘large,’ the work function or HOMO energy level has a largerabsolute value from the vacuum energy level (0 eV), while when the workfunction, HOMO energy level, or LUMO energy level is ‘shallow,’ ‘high,’or ‘small,’ the work function or HOMO energy level has a smallerabsolute value from the vacuum energy level (0 eV).

First, referring to FIG. 1, a schematic structure of anelectroluminescent device according to an embodiment is described.

FIG. 1 is a schematic cross-sectional view of an electroluminescentdevice according to an embodiment.

An electroluminescent device 10 according to an embodiment includes afirst electrode 110 and a second electrode 160 facing each other, anemission layer 140 disposed between the first electrode 110 and thesecond electrode 160 and including a plurality of (e.g., at least two)light emitting particles 141, a hole transport layer 130 disposedbetween the first electrode 110 and the emission layer 140, and anelectron transport layer 150 disposed between the emission layer 140 andthe second electrode 160.

The electroluminescent device 10 according to an embodiment suppliescurrent to the emission layer 140 including light emitting particles 141through the first electrode 110 and the second electrode 160 and causeselectro-luminescence of the light emitting particles 141 to generatelight. The electroluminescent device 10 may generate light in variouswavelength regions according to materials, sizes, or fine structures ofthe light emitting particles 141 of the emission layer 140.

In an embodiment, the first electrode 110 may be directly connected to adriving power source so may function to flow current to the emissionlayer 140. The first electrode 110 may include a material having lighttransmittance in at least a visible light wavelength region but is notlimited thereto. The first electrode 110 may include a material havinglight transmittance in an infrared or ultraviolet (UV) wavelengthregion. For example, the first electrode 110 may be made of an opticallytransparent material.

In an embodiment, the first electrode 110 may include molybdenum oxide,tungsten oxide, vanadium oxide, rhenium oxide, niobium oxide, tantalumoxide, titanium oxide, zinc oxide, nickel oxide, copper oxide, cobaltoxide, manganese oxide, chromium oxide, indium oxide, or a combinationthereof.

Meanwhile, in an embodiment, the first electrode 110 may be disposed onthe substrate 100 as shown in FIG. 1. The substrate 100 may be atransparent insulating substrate or may be made of a ductile material.The substrate 100 may include a glass or a polymer material in a filmhaving a glass transition temperature (T_(g)) of greater than about 150°C. For example, it may include a cycloolefin copolymer (COC) orcycloolefin polymer (COP)-based material.

In an embodiment, the substrate 100 supports the hole transport layer130, the emission layer 140, and the electron transport layer 150sandwiched by the first electrode 110 and the second electrode 160.However, the first electrode 110 of the electroluminescent device 10according to an embodiment is not necessarily disposed on the substrate100, and the substrate may be disposed on the second electrode 160 ormay be omitted as needed.

The second electrode 160 includes an optically transparent material andmay function as a light-transmitting electrode to transmit lightgenerated in the emission layer 140 that will be described later. In anembodiment, the second electrode 160 may include at least one selectedfrom silver (Ag), aluminum (Al), copper (Cu), gold (Au), and an alloythereof, molybdenum oxide, tungsten oxide, vanadium oxide, rheniumoxide, niobium oxide, tantalum oxide, titanium oxide, zinc oxide, nickeloxide, copper oxide, cobalt oxide, manganese oxide, chromium oxide,indium oxide, or a combination thereof.

Each of the first electrode 110 and the second electrode 160 may beformed by depositing a material for forming an electrode on thesubstrate 100 or an organic layer 153 described later by a method suchas sputtering.

The emission layer 140 may include a plurality of light emittingparticles 141 (i.e., at least two light emitting particles). Theemission layer 140 may be formed by applying a resin in which at leasttwo light emitting particles 141 are dispersed on a hole transport layer130 (described later) and curing the same.

The emission layer 140 is a site where electrons and holes transportedby a current supplied from the first electrode 110 and the secondelectrode 160, the electrons and holes are combined in the emissionlayer 140 to generate excitons, and the generated excitons are transitedfrom an excited state to a ground state to emit light in a wavelengthcorresponding to the size of the light emitting particles 141.

In an embodiment, the light emitting particles 141 may include a quantumdot.

The quantum dot has a discontinuous energy bandgap by the quantumconfinement effect. That is, when the emission layer 140 includes aquantum dot as a light emitting particle 141, the emission layer 140 mayproduce light having excellent color reproducibility and color purity.

In an embodiment, a material of the quantum dot is not particularlylimited and known or commercially available quantum dots may be used.For example, the quantum dot may include a Group II-VI compound thatdoes not include Cd, a Group III-V compound, a Group IV-VI compound, aGroup IV element or compound, a Group I-III-VI compound, a GroupI-II-IV-VI compound that does not include Cd, or a combination thereof.The quantum dot according to an embodiment may be a non-cadmium-basedquantum dot. Like this, when the quantum dot comprises or consists of anon-cadmium-cadmium-based material, it has no toxicity compared with aconventional cadmium-based quantum dot and thus is less dangerous and isenvironmentally-friendly.

The Group II-VI compound may be a binary element compound that is ZnS,ZnSe, ZnTe, ZnO, HgS, HgSe, HgTe, MgSe, MgS, or a combination thereof; aternary element compound that is ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe,HgSTe, HgZnS, HgZnSe, HgZnTe, MgZnSe, MgZnS, or a combination thereof;or a quaternary element compound that is HgZnTeS, HgZnSeS, HgZnSeTe,HgZnSTe, or a combination thereof. The Group II-VI compound may furtherinclude a Group III metal.

The Group III-V compound may be a binary element compound that is GaN,GaP, GaAs, GaSb, AlN, AlP, AlAs, AlSb, InN, InP, InAs, InSb, or acombination thereof; a ternary element compound that is GaNP, GaNAs,GaNSb, GaPAs, GaPSb, AlNP, AlNAs, AlNSb, AlPAs, AlPSb, InNP, InNAs,InNSb, InPAs, InPSb, InZnP, or a combination thereof; or a quaternaryelement compound that is GaAlNP, GaAlNAs, GaAlNSb, GaAlPAs, GaAlPSb,GaInNP, GaInNAs, GaInNSb, GaInPAs, GaInPSb, InAlNP, InAlNAs, InAlNSb,InAlPAs, InAlPSb, or a combination thereof. The Group III-V compound mayfurther include a Group II metal (e.g., InZnP).

The Group IV-VI compound may be a binary element compound that is SnS,SnSe, SnTe, PbS, PbSe, PbTe, or a combination thereof; a ternary elementcompound that is SnSeS, SnSeTe, SnSTe, PbSeS, PbSeTe, PbSTe, SnPbS,SnPbSe, SnPbTe, or a combination thereof; or a quaternary elementcompound that is SnPbSSe, SnPbSeTe, SnPbSTe, or a combination thereof.Examples of the Group compound may be CuInSe₂, CuInS₂, CuInGaSe, andCuInGaS, but are not limited thereto. Examples of the Group I-II-IV-VIcompound may be CuZnSnSe and CuZnSnS, but are not limited thereto.Examples of the Group IV compound may be a single substance that is Si,Ge, or a combination thereof; or a binary element compound that is SiC,SiGe, or a combination thereof.

The binary element compound, the ternary element compound, or thequaternary element compound respectively may exist in a uniformconcentration in the particle or in partially different concentrationsin the same particle.

According to an embodiment, the quantum dot may have a core-shellstructure including one semiconductor nanocrystal core and anothersemiconductor nanocrystal shell surrounding the core. The core and theshell may have a concentration gradient wherein the concentration of theelement(s) of the shell decreases toward the core. In addition, thequantum dot may have one semiconductor nanocrystal core and multipleshells surrounding the core. Herein, the multi-layered shell structurehas a structure of two or more shells and each layer may have a singlecomposition or an alloy, or may have a concentration gradient.

When the quantum dot according to an embodiment has a core-shellstructure, a material composition of the shell has a larger energybandgap than that of the core, which may exhibit an effective quantumconfinement effect. However, the embodiment is not limited thereto.Meanwhile, in the multi-layered shell, a shell that is outside of thecore has may have a higher energy bandgap than a shell that is near tothe core and the quantum dot may have ultraviolet (UV) to infraredwavelength ranges.

The quantum dot may have an external quantum efficiency (EQE) of greaterthan or equal to about 10%, for example, greater than or equal to about30%, greater than or equal to about 50%, greater than or equal to about60%, greater than or equal to about 70%, greater than or equal to about90%, or even 100%.

In a display, the quantum dot may have a relatively narrow spectrum soas to improve color purity or color reproducibility. The quantum dot mayhave for example a full width at half maximum (FWHM) of a wavelengthspectrum of less than or equal to about 45 nm, less than or equal toabout 40 nm, or less than or equal to or about 30 nm. Within the ranges,color purity or color reproducibility of a device may be improved.

The quantum dot may have an average particle diameter (the longestdiameter for a non-spherically shaped particle) of about 1 nm to about100 nm. For example, the quantum dot may have an average particlediameter of about 1 nm to about 20 nm, for example, about 2 nm (or about3 nm) to about 15 nm.

In addition, a shape of the quantum dot particle may be general shapesin this art and thus may not be particularly limited. For example, thequantum dot particle may have a spherical, oval, tetrahedral, pyramidal,cuboctahedral, cylindrical, polyhedral, multi-armed, or cubenanoparticle, nanotube, nanowire, nanofiber, nanosheet, or a combinationthereof. The quantum dot may have any cross-sectional shape.

The quantum dot may be commercially available or may be synthesized inany method. For example, several nano-sized quantum dots may besynthesized according to a wet chemical process. In the wet chemicalprocess, precursors react in an organic solvent to grow crystalparticles, and the organic solvent or a ligand compound may coordinatethe surface of the quantum dot, controlling the growth of the crystal.Examples of the organic solvent and the ligand compound are known. Theorganic solvent coordinated on the surface of the quantum dot may affectstability of a device, and thus excess organic materials that are notcoordinated on the surface of the nanocrystals may be removed by pouringit in excessive non-solvent and centrifuging the resulting mixture.Examples of the non-solvent may be acetone, ethanol, methanol, and thelike, but are not limited thereto. After the removal of excess organicmaterials, the amount of the organic materials coordinated on thesurface of the quantum dot may be less than or equal to about 50% byweight (wt %), for example, less than or equal to about 30 wt %, lessthan or equal to about 20 wt %, or less than or equal to about 10 wt %based on a weight of the quantum dot. The organic material may include aligand compound, an organic solvent, or a combination thereof.

The quantum dot may have for example an organic ligand having ahydrophobic moiety bound to its surface. In an embodiment, the organicligand having the hydrophobic moiety may be RCOOH, RNH₂, R₂NH, R₃N, RSH,R₃PO, R₃P, ROH, RCOOR′, RPO(OH)₂, R₂POOH (wherein, R and R′ areindependently a C5 to C24 alkyl group, a C5 to C24 alkenyl group, a C5to C20 alicyclic group, or a C5 to C20 aryl group), a polymeric organicligand, or a combination thereof. The organic ligand may be amono-functional group organic ligand and the functional group may bebound to the surface of the quantum dot.

In an embodiment, the hole transport layer 130 may be disposed betweenthe first electrode 110 and the emission layer 140. The hole transportlayer 130 may transport holes into the emission layer 140.

The electroluminescent device 10 according to an embodiment may furtherinclude a hole injection layer 120 between the hole transport layer 130and the first electrode 110. The hole injection layer 120 supplies holesto the hole transport layer 130.

Each of the hole injection layer 120 and the hole transport layer 130may include, independently, for examplepoly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS),poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine) (TFB), apolyarylamine, poly(N-vinylcarbazole), a polyaniline, a polypyrrole,N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine (TPD),4-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (α-NPD), m-MTDATA(4,4,4″-tris[phenyl(m-tolyl)amino]triphenylamine),4,4′,4″-tris(N-carbazolyl)-triphenylamine (TCTA),1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC), or a combinationthereof, but is not limited thereto. Various semiconductor materials, orcombinations thereof may be applied according to an internal energylevel of the electroluminescent device 10.

The hole injection layer 120 and the hole transport layer 130 accordingto an embodiment may be formed by coating a solution-type commerciallyavailable materials on the first electrode 110 and then curing the samebut is not limited thereto.

In an embodiment, the electron transport layer 150 is disposed betweenthe emission layer 140 and the second electrode 160 and transportselectrons into the emission layer 140.

The electron transport layer 150 according to an embodiment may compriseor consist of a non-light-emitting charge transporting material thatdoes not emit light by an electric field. Thereby, internal lightemission of the electroluminescent device 10 according to an embodimentoccurs in the emission layer 140 and not in the electron transport layer150.

The electron transport layer 150 according to an embodiment may includean inorganic layer 151 formed on the emission layer 140 and including aplurality of inorganic nanoparticles 152 (i.e., two or more inorganicnanoparticles) and an organic layer 153 formed directly on the inorganiclayer 151 on a side opposite the emission layer 140.

As shown in FIG. 1, the inorganic layer 151 may directly be formed onthe emission layer 140.

The inorganic layer 151 may include two or more (i.e., plurality of)inorganic nanoparticles 152 and the two or more inorganic nanoparticles152 may be agglomerated with each other to form a cluster layer. In anembodiment, the inorganic layer 151 may include a cluster layer composedof two or more inorganic nanoparticles 152. In an embodiment, theinorganic layer 151 may be a cluster layer composed of two or moreinorganic nanoparticles 152 (accordingly, the inorganic layer 151 may beexpressed as cluster layer 151 in FIG. 1)

In the case where the cluster layer composed of two or more inorganicnanoparticles 152 is directly on the emission layer 140, thenon-emission quenching of the charge exchange excitons generated in theemission layer 140 may be reduced or prevented, and the luminance of theemission layer 140 may be improved.

The inorganic nanoparticles 152 according to an embodiment may includeinorganic oxide nanoparticles including ZnO, TiO₂, ZrO₂, SnO₂, WO₃,Ta₂O₃, or a combination thereof, but are not limited thereto.

In the case where the cluster layer is made of inorganic oxidenanoparticles, for example, the emission layer 140 including anon-cadmium quantum dot generates a large amount of charge exchangeexcitons due to internal heat generation during driving of the device,and the generated charge exchange exciton may emit auger electrons onthe interface with the cluster layer without non-emission quenching bythe cluster layer. The emitted auger electrons may emit light byrecombination with holes in the emission layer 140, roll-off of theelectroluminescent device 10 in a high luminance region may beminimized.

On the other hand, an average particle diameter (as for anon-spherically shaped particle, diameter means the longest dimension)of the inorganic nanoparticles 152 according to an embodiment may be forexample less than or equal to about 150 nm, less than or equal to about140 nm, less than or equal to about 130 nm, less than or equal to about120 nm, less than or equal to about 110 nm, less than or equal to about100 nm, less than or equal to about 90 nm, less than or equal to about80 nm, less than or equal to about 70 nm, less than or equal to about 60nm, less than or equal to about 50 nm, less than or equal to about 40nm, less than or equal to about 30 nm, less than or equal to about 20nm, or less than or equal to about 10 nm and for example greater than orequal to about 1 nm, greater than or equal to about 2 nm, greater thanor equal to about 3 nm, greater than or equal to about 4 nm, or greaterthan or equal to about 5 nm.

When the inorganic layer 151 includes the cluster layer composed of twoor more inorganic nanoparticles 152 as described above, electronmobility thereof is much higher than that of a general inorganicsemiconductor film or an organic semiconductor film. Thereforeluminescence stability and the luminance of the non-cadmium quantum dotmay be improved through the cluster layer to which the inorganic layer151 is applied.

In addition, an average thickness of the cluster layer according to anembodiment may be for example less than or equal to about 100 nm, lessthan or equal to about 90 nm, less than or equal to about 80 nm, lessthan or equal to about 70 nm, less than or equal to about 60 nm, lessthan or equal to about 50 nm, or less than or equal to about 40 nm andfor example greater than or equal to about 10 nm, greater than or equalto about 15 nm, greater than or equal to about 20 nm, greater than orequal to about 25 nm, or greater than or equal to about 30 nm.

When the average particle diameter of the inorganic nanoparticles 152and the average thickness of the cluster layer are within the aboveranges, the inorganic layer 151 may exhibit excellent electron mobility.

FIG. 2 is a microscopic image showing the upper surface of the inorganiclayer of the electroluminescent devices according to an embodiment andFIG. 3 is a graph showing an upper surface three-dimensional shape imageand a height deviation depending on a position of an inorganic layer anda position of the inorganic layer of the electroluminescent deviceaccording to an embodiment measured using a Zygo interferometer.

FIGS. 2 and 3 correspond to a case where a cluster layer having athickness of about 40 nm is formed by agglomerating ZnO nanoparticleshaving an average particle diameter of 3 nm.

First, referring to FIG. 2, it may be seen that the surface of thecluster layer includes two or more grains composed of inorganicnanoparticles and a grain boundary formed between neighboring grains.

In addition, referring to the graph showing the surface roughness curveof the linear portion indicated in the surface profile image of thecluster layer of FIG. 3, a peak to valley (PV) of the surface of thecluster layer is 16.343 nm and a root mean square roughness (Rq)calculated by a root mean square (rms) method is 1.914 nm.

Thus, referring now to FIG. 1, the surface region 151 a of the clusterlayer according to an embodiment may exhibit an uneven surfacemorphology, which may be caused by a non-uniform agglomeration of theinorganic nanoparticles 152 a in the surface region 151 a, a particlesize variation of the inorganic nanoparticles 152 a in the surfaceregion 151 a, detachment of the inorganic nanoparticles 152 a from thecluster layer, or the like, or a combination thereof, which results inthe existence of grain boundaries in the surface region 151 a. In someembodiments, the upper surface of the cluster layer includes the surfaceregion 151 a having two or more grains each comprising inorganicnanoparticles, wherein a grain boundary is formed between adjacentgrains. The inorganic nanoparticles in the surface region 151 a caninclude inorganic nanoparticles 152 a. In other words, the inorganicnanoparticles 152 a are inorganic nanoparticles of the plurality ofnanoparticles that are not included in the cluster layer.

On the other hand, according to an embodiment, the organic layer 153 isformed directly on at least a portion of the inorganic layer 151. Theorganic layer 153 may be formed through a deposition process or the likeon the cluster layer of the inorganic nanoparticles 152.

The organic layer 153 is a layer composed of organic semiconductorcompounds and may include conductive monomolecular molecule, a lowmolecular organic nano-material having a conjugation structure, or acombination thereof. Specific examples of the organic semiconductorcompounds forming the organic layer 153 may be a quinolone-basedcompound, a triazine-based compound, a quinoline-based compound, atriazole-based compound, a naphthalene-based compound, or a combinationthereof, but are not limited thereto. In some embodiments, the organicsemiconductor compound can be phosphine oxide based compound (NET-218,Novaled Electron Transport material 218, obtained from Novaled),phosphonyl phenol based compound (NDN-87, Novaled Dopant n-side material87, obtained from Novaled), or a combination thereof.

The organic layer 153 may be composed of an organic semiconductorcompound having a higher work function than that of the inorganic layer151. Accordingly, the organic layer 153 may be an energy barrier againstelectrons moving from the second electrode 160 to the inorganic layer151.

A LUMO (Lowest Unoccupied Molecular Orbital) energy level of the organiclayer 153 may be for example about −1.8 electron Volts (eV) to about−2.8 eV, about −1.8 eV to—about 2.7 eV, about −1.8 eV to about −2.6 eV,about −1.8 eV to about −2.5 eV,—about 1.8 eV to about −2.4 eV, about−1.8 eV to about −2.3 eV, or about −1.8 eV to about −2.2 eV.

On the other hand, the organic layer 153 may have for example, electronmobility of greater than or equal to about 10⁻⁴ square centimeters perVolt-second (cm²/V·s) or greater than or equal to about 10⁻³ cm²/V·s,and for example, less than or equal to about 1 cm²/V·s or less than orequal to about 10⁻¹ cm²/V·s.

The organic layer 153 according to an embodiment fully or completelycovers the upper surface of the inorganic layer 151, so that theinorganic layer 151 may not be exposed toward the second electrode 160as shown in FIG. 1. In other words, the organic layer 153 is disposed onand in contact with the entire upper surface of the inorganic layer 151.Accordingly, the organic layer 153 fills at least a part or a whole of agrain boundary formed at, on, or in the surface (i.e., in the surfaceregion 151 a between the inorganic nanoparticles 152 a) of the clusterlayer being in contact therewith.

On the other hand, the organic layer 153 may fill a part of or all ofthe cracks or voids generated by detachment or non-uniform agglomerationof the inorganic nanoparticles 152 a on the surface of the cluster layerbeing in contact therewith. In some embodiments, the organic layer 153is disposed on the upper surface of the inorganic layer 151 such thatsubstantially all of the cracks, voids, grain boundaries, or the like,at the upper surface are filled with or include the semiconductororganic compounds of the organic layer 153.

A thickness of the organic layer 153 may be variously designed dependingon electron mobility, a work function, and the like of the organic layer153. It may be thin enough to transport at least electrons throughtunneling to the inorganic layer 151 while it may be thick enough tofill a grain boundary, a crack, and a void on the surface of theinorganic layer 151 and not expose but cover the inorganic layer 151.

In an embodiment, a thickness of the organic layer 153 may be forexample less than or equal to about 20 nm, less than or equal to about18 nm, less than or equal to about 16 nm, less than or equal to about 14nm, less than or equal to about 12 nm, less than or equal to about 10nm, less than or equal to about 8 nm, less than or equal to about 6 nm,or less than or equal to about 4 nm and for example greater than orequal to about 2 nm, greater than or equal to about 2.5 nm, or greaterthan or equal to about 3 nm.

Since the organic layer 153 has a higher work function than theinorganic layer 151, a driving voltage for transporting electrons may beincreased compared with a case of using the inorganic layer alone as anelectron transport layer. However, when the organic layer 153 accordingto an embodiment has a LUMO and an electron mobility within the rangeand in addition, a thickness adjusted within the range, the drivingvoltage and an electron mobility rate may be adjusted to have anappropriate level due to band bending and tunneling effects.

On the other hand, the organic layer 153 may include at least twodifferent organic semiconductor compounds. The organic layer 153according to an embodiment may be formed, for example, in a method ofco-depositing at least two different organic semiconductor compounds, orthe like.

For example, when either one of the organic semiconductor compounds isused, a plurality of island-shaped intermediates is formed due to unevenmorphology on the surface of the inorganic layer 151 during theformation of the organic semiconductor compound, and accordingly, theorganic layer 153 may have a large thickness deviation. On the otherhand, when at least two different organic semiconductor compounds areused, the organic layer 153 may have a relatively small thicknessdeviation.

In addition, the organic layer 153 may include two different organicsemiconductor compounds (a first organic semiconductor compound and asecond organic semiconductor compound).

Herein, the first organic semiconductor compound and the second organicsemiconductor compound in the organic layer 153 may be used in variousweight ratios depending on materials of each organic semiconductorcompound, wherein the weight ratio may be adjusted to minimize formationof the island-shaped intermediate.

In an embodiment, the first organic semiconductor and the second organicsemiconductor in the organic layer 153 may, for example, have a weightratio of about 2:8 to about 8:2, about 3:7 to about 7:3, about 4:6 toabout 6:4, or about 5:5.

On the other hand, additional units including the inorganic layer 151and the organic layer 153 may be alternately stacked on the emissionlayer 140. In other words, the electron transport layer 150 may includeplurality of stacked units, such as at least two stacked units, witheach unit including the inorganic layer 151 and the organic layer 153.In this way, the number of alternately stacking units each including theinorganic layer 151 and the organic layer 153 may be adjusted toappropriately maintain a charge carrier balance in an electroluminescentdevice by considering each energy level of the first electrode 110, thesecond electrode 160, the hole injection layer 120, the hole transportlayer 130, and the emission layer 140, a difference of their energydifferences, and the like.

On the other hand, when the cluster layer composed of the inorganicnanoparticles is applied to an emission layer including quantum dots,device efficiency at high luminance may be secured, but since a crack, avoid, a grain boundary, or the like on the surface of the cluster layermay for example result in a leakage path of a current, a leakage currentfrom the leakage path may deteriorate device efficiency at a low voltageand in a low luminance region.

However, the electroluminescent device 10 according to an embodimentincludes the organic layer 153 filling the crack, the void, the grainboundary, or the like, on the surface of the cluster layer and thus mayminimize the generation of the leakage current due to the cluster layer.

The electroluminescent device 10 according to an embodiment may furtherinclude an electron injection layer (not shown) between the secondelectrode 160 and the electron transport layer 150 and may furtherinclude an electron blocking layer (not shown) between the secondelectrode 160 and the electron injection layer (not shown) or betweenthe electron injection layer (not shown) and the electron transportlayer 150. However, the embodiment is not limited thereto and theelectron injection layer (not shown) and the electron blocking layer(not shown) may be omitted in order to maintain charge carrier balanceof the electroluminescent device 10 to be an appropriate level.

Hereinafter, referring to FIG. 4, a driving principle of anelectroluminescent device according to an embodiment is explained.

FIG. 4 is an energy band diagram showing an electroluminescent deviceaccording to an embodiment.

The electroluminescent device 10 according to an embodiment includesquantum dots as the light emitting particles 141, and accordingly, theemission layer 140 formed thereof has a different energy level from thatof a general organic light emitting diode.

In particular, the general electroluminescent device sequentiallytransports electrons along with a LUMO energy level from an electrodethrough an electron transport layer toward an emission layer, but theelectroluminescent device 10 according to an embodiment has the organiclayer 153 having a higher LUMO energy level than that of the inorganiclayer 151 and thus working as a high energy barrier. Accordingly, theelectron transport layer 150 according to an embodiment has a hybridstructure consisting of the inorganic layer 151 as a cluster layerformed of inorganic nanoparticles and the organic layer 153 covering theinorganic layer 151.

In an embodiment, when the cluster layer formed of inorganicnanoparticles is formed on the surface of the emission layer 140including quantum dots, this cluster layer has very high electronmobility relative to hole mobility between the hole injection layer(HIL) 120 and the hole transport layer (HTL) 130, and a leakage currentmay be generated due to uneven surface morphology of the cluster layer.

Accordingly, in an embodiment, the organic layer 153 having a higherwork function than that of the cluster layer may be formed to cover thesurface of the cluster layer to minimize the leakage current generatedfrom the cluster layer. In addition, the organic layer 153, as shown inFIG. 4, simultaneously works as a kind of an energy barrier and thus mayadjust entire electron mobility of the electron transport layer 150 intoan appropriate level and thus improve a charge carrier balance.

As described above, the electroluminescent device 10 according to anembodiment includes the inorganic layer 151 having very high electronmobility and improving luminous efficiency of the emission layer 140including quantum dots and the organic layer 153 minimizing the leakagecurrent due to the surface morphology of the inorganic layer 151 andsimultaneously, adjusting entire electron mobility of the electrontransport layer 150 into an appropriate level, and accordingly, theelectron transport layer 150 has a hybrid stacking structure of theinorganic layer 151/the organic layer 153.

Accordingly, the electroluminescent device 10 according to an embodimentmay improve a charge carrier balance and light extraction efficiency ofan emission layer and also, minimize a leakage current and thus showexcellent driving characteristics and life-span characteristics.

A display device according to an embodiment including theelectroluminescent device 10 is described.

A display device according to an embodiment includes a substrate, adriving circuit formed on the substrate, and a first electroluminescentdevice, a second electroluminescent device, and a thirdelectroluminescent device spaced apart from each other in apredetermined interval and disposed on the driving circuit.

The first to third electroluminescent devices have the same structure asthe electroluminescent device 10 and but the wavelengths of the lightemitted from each of the quantum dots may be different from each other.

In an embodiment, the first electroluminescent device is a red deviceemitting red light, the second electroluminescent device is a greendevice emitting green light, and the third electroluminescent device isa blue device emitting blue light. In other words, the first to thirdelectroluminescent devices may be pixels expressing (i.e., emitting)red, green, or blue colored light, respectively, in the display device.

However, an embodiment is not necessarily limited thereto, and the firstto third electroluminescent devices may respectively express magenta,yellow, or cyan, or may express other colors.

One or more of the first to third electroluminescent devices may be theelectroluminescent device 10. For example, an electroluminescent devicedisplaying blue in the display device is the electroluminescent device10 and electroluminescent devices displaying red and green may includean electron transport layer that consists of an organic layer orinorganic layer or that includes both organic layer and inorganic layer,provided that an organic layer is formed directly on the emission layer.Alternatively, one of the first to third electroluminescent devices maybe the electroluminescent device 10 and the rest may beelectroluminescent devices including a fluorescent material or aphosphor material instead of the quantum dot as the light emittingparticle.

The substrate may be a transparent insulating substrate or may be madeof a ductile material. The substrate may include glass or a polymermaterial in a film having a glass transition temperature (T_(g)) ofgreater than about 150° C. For example, it includes a cycloolefincopolymer (COC) or cycloolefin polymer (COP) based material.

The driving circuit is disposed on the substrate and is independentlyconnected to each of the first to third electroluminescent devices. Thedriving circuit may include at least one set of lines including a scanline, a data line, a driving power source line, a common power sourceline, and the like, at least two of thin film transistors (TFT)connected to the wire and corresponding to one organic light emittingdiode, and at least one capacitor, or the like. The driving circuit mayhave a variety of the known structures.

As described above, a display device according to an embodiment maydisplay images having improved color purity and color reproducibilitywithout separate light source such as a backlight unit, and particularlymay exhibit improved driving characteristics and life-spancharacteristics.

Hereinafter, the embodiments are illustrated in more detail withreference to examples. However, these examples are exemplary, and thepresent disclosure is not limited thereto.

EXAMPLES Example 1

An indium time oxide (ITO) layer is deposited on a glass substrate as afirst electrode (anode), and a PEDOT:PSS layer (hole injection layer)and a poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine)(TFB) polymer layer (hole transport layer) are respectively andsequentially formed thereon by using a solution process. Subsequently, ablue emission layer is formed thereon by coating blue quantum dots(ZnTeSe) dispersed in an organic solvent and heat-treating it under anitrogen atmosphere at 80° C. for 30 minutes.

On the other hand, ZnO particles having an average particle diameter of3 nm are three times washed and then, formed into a ZnO cluster layer(inorganic layer) having a thickness of about 40 nm on the blue emissionlayer.

Subsequently, on the surface of the ZnO cluster layer, an organic layerwhich includes organic compound 1 (a phosphine oxide compound, NovaledElectron Transport material 218 (NET-218), obtained from Novaled)(organic layer) is formed to be 2 nm to 20 nm thick at a deposition rateof 0.5 Å/s to 1.0 Å/s, and an Al layer is deposited thereon tomanufacture an electroluminescent device according to Example 1.

Example 2

An electroluminescent device is manufactured according to the samemethod as Example 1 except that the organic compound 1 and an organiccompound 2 (phosphonyl phenol compound, Novaled Dopant n-side material87 (NDN-87), obtained from Novaled) in a weight ratio of 1:1 areco-deposited to form an organic layer having the organic compound1:organic compound 2 blend instead of organic compound 1.

Example 3

An electroluminescent device is manufactured according to the samemethod as Example 1, except that the ZnO cluster layer is formed to havea thickness of 20 nm.

Comparative Example 1

An electroluminescent device is manufactured according to the samemethod as Example 1, except that the Al layer is formed directly on theZnO cluster without forming the organic layer.

Comparative Example 2

An electroluminescent device is manufactured according to the samemethod as Example 1, except that a ZnO:organic compound 1 blend layer isformed on the blue emission layer by twice to three times washing theZnO particles having an average particle diameter of 3 nm and thenmixing them with the organic compound 1 in a weight ratio of 20:1, andthen, the Al layer is deposited thereon.

Comparative Example 3

An electroluminescent device is manufactured according to the samemethod as Comparative Example 2, except that a ZnO:organic compound 2blend layer is formed by mixing ZnO and the organic compound 2 in aweight ratio of 20:1, instead of using the organic compound 1.

Comparative Example 4

An electroluminescent device is manufactured according to the samemethod as Example 1, except that the Al layer is formed directly on theZnO cluster without forming the organic layer.

Characterization

First, I-V-L characteristics of the electroluminescent devices accordingto Example 1 and Comparative Examples 1 to 3 are respectively shown inFIGS. 5 to 7, and J-V characteristics thereof are shown in FIG. 8.

FIGS. 5 to 7 are graphs showing I-V-L characteristics of theelectroluminescent devices according to Example 1 and ComparativeExamples 1 to 3; FIG. 5 shows voltage-current density, FIG. 6 showsvoltage-luminance, and FIG. 7 shows luminance-external quantumefficiency (EQE), respectively.

Referring to FIGS. 5 and 6, Example 1 shows a lower current density atthe same voltage and a lower luminance at the same voltage compared withComparative Examples 1 and 2, and the reason is that the organic layerincluding organic compound 1 works as an energy barrier and decreasescurrent density at a lower voltage.

However, referring to FIG. 7, Example 1 shows very excellent externalquantum efficiency at luminance of less than or equal to about 1000cd/m² compared with the Comparative Examples. In particular, Example 1shows the best external quantum efficiency (about 1.7%) at a luminanceof about 100 cd/m². In addition, Example 1 shows a smaller externalquantum efficiency decrease at a higher voltage in a higher luminanceregion compared with the Comparative Examples, and thus provides stabledriving characteristics.

In other words, since the organic layer including organic compound 1 isfurther formed on the ZnO cluster layer and thus works as an energybarrier against electrons moving from the Al layer to the ZnO clusterlayer, Example 1 has a slight decrease in current density and luminancebut shows excellent luminous efficiency at low luminance.

On the other hand, Comparative Example 3 shows lower current density atthe same voltage, lower luminance at the same voltage, and lowerexternal quantum efficiency at the same luminance compared with Example1 as well as Comparative Example 2, and these results are expected basedon a difference of organic semiconductor materials.

FIG. 8 is a graph showing a voltage-current density relationship of theelectroluminescent devices according to Example 1 and ComparativeExamples 1 to 3. FIG. 8 is a graph showing the current density in they-axis on a logarithmic scale.

Referring to FIG. 8, Example 1 has an inorganic-organic hybrid stackingstructure by further forming the organic layer including organiccompound 1 on the surface of the ZnO cluster layer and thus shows about1,000 times less current leakage compared with Comparative Example 1including only the ZnO cluster layer and no organic layer.

On the other hand, when electroluminescence intensity relative to awavelength is examined regarding the electroluminescent device ofExample 1, the electroluminescent device of Example 1 shows a narrowfull width at half maximum (FWHM) of about 28 nm.

The electroluminescent devices according to Example 2 and ComparativeExamples 1 and 2 are measured regarding a luminance decrease dependingon time, and the results are shown in FIG. 9.

FIG. 9 is a time-luminance graph of the electroluminescent devicesaccording to Example 2, Comparative Example 1, and Comparative Example2.

Referring to FIG. 9, the electroluminescent device of Example 2 shows aslower rate of luminance decrease compared with those of ComparativeExamples 1 and 2, and when T₅₀ is defined as a time when anelectroluminescent device shows 50% luminance relative to initialluminance, Example 2 shows the longest T₅₀ of 1.42 hours, ComparativeExample 1 shows T₅₀ of 0.27 hours, and Comparative Example 2 shows T₅₀of 0.74 hours.

The life-span characteristics decrease as a leakage path due to surfacemorphology of the ZnO cluster layer increases, and accordingly, Example2 including the organic layer including the organic compound 1:organiccompound 2 blend covering and minimizing the leakage path of the ZnOcluster layer shows excellent life-span characteristics compared withthe Comparative Examples.

On the other hand, I-V-L characteristics of the electroluminescentdevices of Example 3 and Comparative Example 4 are respectively shown inFIGS. 10 to 12, and J-V characteristics are shown in FIG. 13.

FIGS. 10 to 12 are graphs showing I-V-L characteristics of theelectroluminescent devices according to Example 3 and ComparativeExample 4; FIG. 10 shows voltage-current density, FIG. 11 showsvoltage-luminance, and FIG. 12 shows luminance-external quantumefficiency, respectively.

Referring to FIGS. 10 to 12, Example 3 shows a slightly lower currentdensity at the same voltage and a slightly lower luminance at the samevoltage, since the organic layer including the organic compound1:organic compound 2 blend works as an energy barrier against electronsmoving from the Al layer to the ZnO cluster, and also a very highluminance-external quantum efficiency of about 2.6% at a lower luminanceof less than or equal to about 500 cd/m² and particularly, less than orequal to about 100 cd/m².

FIG. 13 is a voltage-current density graph of the electroluminescentdevices according to Example 3 and Comparative Example 4. FIG. 13 is agraph showing the current density in the y-axis on a logarithmic scale.

Referring to FIG. 13, Example 1 further having the organic layerincluding the organic compound 1:organic compound 2 blend on the surfaceof the ZnO cluster layer shows an excellent reduction in leakage currentcompared with Comparative Example 4 having only the ZnO cluster layerwithout an organic layer.

While this disclosure has been described in connection with what ispresently considered to be practical example embodiments, it is to beunderstood that the bounds of this disclosure are not limited to thedisclosed embodiments. On the contrary, it is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. An electroluminescent device, comprising: a firstelectrode and a second electrode facing each other; an emission layerdisposed between the first electrode and the second electrode andcomprising a plurality of light emitting particles; a hole transportlayer disposed between the first electrode and the emission layer; andan electron transport layer disposed between the emission layer and thesecond electrode, wherein the electron transport layer comprises: aninorganic layer disposed on the emission layer, the inorganic layercomprising a plurality of inorganic nanoparticles; and an organic layerdirectly disposed on at least a portion of an upper surface of theinorganic layer on a side opposite the emission layer, wherein theorganic layer has a lowest unoccupied molecular orbital (LUMO) energylevel of about −1.8 electron Volts to about −2.8 electron Volts, andwherein an average thickness of the organic layer is about 2 nanometersto about 20 nanometers.
 2. The electroluminescent device of claim 1,wherein the organic layer completely covers the upper surface of theinorganic layer between the inorganic layer and the second electrode. 3.The electroluminescent device of claim 1, wherein the organic layer hasan electron mobility of about 10⁻³ square centimeters per Volt-second toabout 10⁻¹ square centimeters per Volt-second.
 4. The electroluminescentdevice of claim 1, wherein an inorganic nanoparticle of the plurality ofinorganic nanoparticles is ZnO, TiO₂, ZrO₂, SnO₂, WO₃, Ta₂O₃, or acombination thereof.
 5. The electroluminescent device of claim 1,wherein an average particle diameter of the plurality of inorganicnanoparticles is less than or equal to about 150 nanometers.
 6. Theelectroluminescent device of claim 1, wherein the inorganic layer isdisposed on the emission layer as a cluster layer comprising theplurality of inorganic nanoparticles.
 7. The electroluminescent deviceof claim 6, wherein the cluster layer is disposed directly on theemission layer.
 8. The electroluminescent device of claim 6, wherein anupper surface of the cluster layer comprises a surface region having twoor more grains each comprising the inorganic nanoparticles, wherein agrain boundary is formed between adjacent grains, and the organic layerfills at least a portion of the grain boundary.
 9. Theelectroluminescent device of claim 1, wherein the organic layercomprises an organic semiconductor compound that is a quinolone-basedcompound, a triazine-based compound, a quinoline-based compound, atriazole-based compound, a naphthalene-based compound, or a combinationthereof.
 10. The electroluminescent device of claim 9, wherein theorganic layer comprises at least two different organic semiconductorcompounds.
 11. The electroluminescent device of claim 10, wherein theorganic layer comprises a first organic semiconductor compound and asecond organic semiconductor compound that are different from eachother, and a weight ratio of the first organic semiconductor compoundand the second organic semiconductor compound in the organic layer isabout 3:7 to about 7:3.
 12. The electroluminescent device of claim 1,wherein the electron transport layer further comprises at least oneadditional unit comprising a second inorganic layer and a second organiclayer, wherein the at least one additional unit is alternately stacked.13. The electroluminescent device of claim 1, wherein the plurality oflight emitting particles comprises quantum dots.
 14. Theelectroluminescent device of claim 13, wherein the quantum dots are aGroup II-VI compound that does not comprise Cd, a Group III-V compound,a Group IV-VI compound, a Group IV element or compound, a Group I-III-VIcompound, a Group I-II-IV-VI compound that does not comprise Cd, or acombination thereof.
 15. The electroluminescent device of claim 13,wherein the quantum dots have a core-shell structure.
 16. Theelectroluminescent device of claim 1, wherein the electron transportlayer does not have electroluminescence.
 17. A display device comprisingthe electroluminescent device of claim 1.